Definitely could be a great game, but suffers from monotonous mistakes
Story: (may contain spoilers)
The most surprising addition in Brawl was the story mode, which got people wiled up in curiosity. It's not much of a story to write home about so don't get your hopes up. An evil entity known as Tabuu is attempting to pull all known realspace in the the Nintendo universe into the subspace(subspace emissary eh? no >.>). The game doesn't do a good job at all explaining his movies in the slightest so the player is just left to assume that Tabuu is just doing it because he's evil(Several Smash bros sites have indicated that Tabuu was angered by Mario's sparing of kirby or vice versa as he is the lord of the Smash universe or something like that). After which the stadium is attacked by Tabuu's forces which are promptly dispatched easily. Petey Piranha then shows up with the two princesses captured, but is also defeated. Two R.O.B. Robots then activate these bombs tearing the fabric of reality in nuclear like explosions and absorbing all nearby realspace energy, but not before our heroes escape.
The Subspace generally then continues through the perspective of other character;s we meet in either neutral of funny ways. At certain points Nintendo attempts to evoke emotion but falls flat(save for one) on their faces as Nintendo never truly harms any of it's characters in any way(again save for one occasion). The story does a rather competent job of stitching meetings together between characters and locations and captures the personality of each character quite well. After loads of more cutscenes featuring character combat and realizations, they go off to confront tabuu, win and stare into the sunset with Nobuo Uematsu's composed main theme going off.
Overall the story is lackluster and doesn't add any real value to the game. The cutscenes look mediocre as well. For the time it may have been a good contender, but even before the wii, there were much better CG cutscenes on game consoles being put out.
Gameplay:
The primary way to unlock characters and stages, is to play the Subspace emissary story mode. Most missions are well paced and easy enough to progress through but hard enough to keep you on your toes just a bit. The most common enemy you will see will be Tabuu's little spawn like creatures. This would be great if there was some variety to them, but there isn't. Most of those enemies will just be different colored versions of previous enemies encountered, making playing somewhat repetitive and dull.
Beating the main story still does not relinquish all characters however, those can further be unlocked through challenges provided by brawl(such as defeating enemies in a certain order or completing so many challenge stages). These can serve as a much more active challenging gamestyle compared to the main game. You have additional survival periods of 3-5-15-till you die game modes as well. Most characters in the game are fairly balanced, but those who are not will easily stick out(*cough*METAKNIGHT*cough*).
Online is available for the first time in this series to great initial fanfare anticipation and actual disappointment. Getting into any online battle can take ages(and I have a 7Mb/s connection) and the connection is somewhat shaky as well further killing enjoyment. This is critical for those who do not have friends over frequently because the single player mode at that point just become horribly stale.
In the end, some technical flaws, mediocre CG and pseudo impressive cutscenes, and overall repetitive enemy and level designs take it's toll on Brawl. It's a good game, but nowhere near the level that other's make it out to be.